1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for efficiently performing initial access in a multi-carrier broadband wireless access system.
2. Discussion of the Related Art
An Advanced Mobile Station (AMS) and an Advanced Base Station (ABS) communicate with each other on a plurality of carriers in compliance with the Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard, more efficiently than in a single-carrier environment. For example, from the viewpoint of the AMS, it can use a wider bandwidth, whereas from the viewpoint of the ABS, it can accommodate more users. Besides IEEE 802.16m, other radio technologies (e.g. Long Term Evolution-Advanced (LTE-A) support the multi-carrier communication.
A conventional multi-carrier wireless communication system will be described below.
FIG. 1 illustrates examples of using multiple carriers in a conventional wireless communication system.
Generally, contiguous multiple carriers may be aggregated as illustrated in FIG. 1(a) or non-contiguous multiple carriers may be aggregated as illustrated in FIG. 1(b). A carrier aggregation unit is a basic bandwidth unit in a legacy system (e.g. LTE is a legacy system for LTE-A and IEEE 802.16e is a legacy system for IEEE 802.16m). In a multi-carrier environment, two types of carriers are typically defined, primary carrier and secondary carrier.
The primary carrier is the carrier used by the ABS and the AMS to exchange traffic and full PHYsical (PHY)/Medium Access Control (MAC) control information. Further, the primary carrier may be used for a general AMS operation such as network entry. Each AMS has a single carrier that it considers to be its primary carrier for initial network entry to a cell.
The secondary carrier is an additional carrier that can be used for exchanging traffic according to the ABS's specific allocation commands and rules, typically received on the primary carrier.
Based on the usage of the primary and/or secondary carrier, the carriers of the multi-carrier system may be classified differently as follows, from the perspective of the ABS.
Fully configured carrier: A carrier for which all control channels including synchronization, broadcast, multicast and unicast control channels are configured. Information and parameters regarding a multi-carrier operation and the other carriers may also be included in the control channels.
Partially configured carrier: A carrier with only an essential control channel configuration to support traffic exchanges in the multi-carrier environment, that is, a carrier on which only downlink data is transmitted and received.
Preferably, the primary carrier is fully configured, whereas the secondary carrier is fully or partially configured depending on channel status and user requirements. When one of fully configured carriers available to a cell is allocated as a primary carrier to an AMS in the cell, the AMS may be controlled through the primary carrier. In addition, the AMS may dynamically use a plurality of secondary carriers. In general, the AMS receives information about a secondary carrier as well as typical control information on the primary carrier, and transmits and receives data on the secondary carrier or primary carrier. A fully configured carrier allocated as a secondary carrier to an AMS may be a primary carrier for another AMS.
The IEEE 802.16m standard regulates that an AMS enters an ABS in accordance with a conventional IEEE 802.16e procedure. Specifically, the AMS synchronizes to the ABS by contention-based Code Division Multiple Access (CDMA) code ranging, and then performs a network entry procedure by exchanging an Advanced Air Interface Ranging Request (RNG-REQ) message and a Ranging Response (AAI RNG-RSP) message with the ABS. Subsequently, the AMS and the ABS may perform a capability negotiation procedure.
FIG. 2 is a flowchart illustrating an initial access method in the conventional wireless communication system and FIG. 3 is a diagram illustrating a signal flow for a ranging procedure between an AMS and an ABS in the conventional wireless communication system.
Referring to FIG. 2, upon power-on, the AMS searches for an ABS to serve by scanning DownLink (DL) channels. Without initial knowledge of the geography and configuration of a network, the AMS scans the frequencies of neighbor ABSs one by one.
After completing every system setting by acquiring Downlink (DL) and UpLink (UL) system information about the detected ABS, the AMS performs a ranging procedure with the ABS, as illustrated in FIG. 3. The AMS acquires UL synchronization by contention-based ranging with the ABS using a CDMA ranging code.
Until the synchronization is completed, the ABS notifies the AMS of parameters to be adjusted by an RNG-RSP message. The status of the RNG-RSP message is set to “continue” during the parameter adjustment, and when the parameter adjustment is completed, the ABS transmits an RNG-RSP message with a status set to “success” to the AMS. When the AMS is fully synchronized to the ABS, it registers to a broadband network and receives services over the broadband network.
If an initial access is performed in the above conventional method in a multi-carrier environment, the AMS should select one of a plurality of carriers supported by the ABS, for the initial access. However, the AMS's access to a specific carrier may be rejected depending on the load statuses of the carriers of the ABS. Then the ABS attempts an access to another carrier, thus causing an unnecessary delay. Accordingly, there exists a need for a method for efficiently performing an initial access in the AMS, taking into account the load statuses of a plurality of carriers supported by the ABS under the multi-carrier environment.